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egister in ARM selects a new circuit on the route or on an alternative route. The repeater which did not receive a call-confirmation signal initiates a retest procedure consisting of transmission of the sequence 1 second stop polarity and 29 seconds start polarity. This continues until either a callconfirmation signal is received or until the maintenance personnel take action in response to an alarm, which is issued when the retest signalling has proceeded for about 3 minutes. Prevention of Character Printing on Clearing of Call With single current, unless special measures are taken, characters may sometimes be printed on the teleprinter of one subscriber in conjunction with the clearing of a call. If the A subscriber clears, characters will be printed on the B subscriber's teleprinter, and vice versa. To avoid this, a special signal is used on the subscriber line which receives a clearing signal (see signal diagram). The breaking of the subscriber's loop, which indicates clearing, is allowed to last for max. 150 ms, after which the idle circuit condition is restored. After the recognition time for the clearing signal, 400-600 ms, the polarity of the subscriber's line is reversed and the current returns to the free line value. International and Intercontinental Signalling In the introduction to Recommendation U J, CCITT stipulates: "the outgoing country should conform to the signalling requirements of the incoming country." This is an important principle and has a great effect on the design of international and intercontinental telex exchanges. The international telex exchange in Madrid, for example, must interwork with the following signalling systems for fully automatic subscriber-controlled international traffic to • Spain, i.e. Spanish national network Type B with keyboard selection, printed service signals and automatic tripping of Spanish B subscriber's answer-back. • Austria, Denmark, Ger- Type B with dial selection but without printed many, Switzerland service signals and without automatic tripping of B subscriber's answer-back in the respective countries, which is instead done from the Madrid centre. ] UK, Portugal Type B with dial selection and printed service signals but without automatic tripping of B subscriber's answer-back, which is instead done from the Madrid centre. • Yugoslavia Type B with keyboard selection, printed service signals and automatic tripping of B subscriber's answer-back. • Italy Type B with keyboard selection, printed service signals, automatic tripping of B subscriber's answer-back together with sending of date and time. 10
• Belgium, Holland, Norway Type A with printed service signals, sending of register identity, automatic tripping of B subscriber's answer-back together with sending of time. • France Type A with printed service signals, sending of day number and time. Tripping of both A and B subscriber's answer-back. • Canary Islands U 20 signalling on ARQ. On an incoming call to the international transit centre in Madrid the outgoing country receives Type B signals. The outgoing country can choose • whether to get the call-confirmation sent and proceed-to select signals as two separate signals or as a combined signal • to send selection signals in the form of teleprinter signals or dial pulses. As will appear from the above, the primary purpose of an international transit centre is to interwork with different countries in accordance with the existing variant types of signalling systems. Signalling Diagrams Signalling on Subscriber Line See fig. 8 for outgoing traffic, i.e. traffic from A subscriber (TxA), fig. 9 for incoming traffic, i.e. traffic to B subscriber (TxB). The signalling diagrams Fig. 8 Signalling on calling subscribers line Terminal exchange Calling subscriber equipment consisting of teleprinter and control unit Subscriber's line which*may be either a single current line or double current line Forward signalling path, i.e. from subscriber to exchange Backward signalling path, i.e. in opposite direction Proceed-to-select signal Selection information, called subscriber's number Time signal, hours and minutes, e.g. 21.10 Called subscriber's answer-back etc. Called subscriber's number 11
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Page 10 and 11: In a new Recommendation U 11, CCITT
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